X-ray diffraction was employed to study the evolution of the charge density wave (CDW) in Cu_{x}TiSe_{2} as a function of copper intercalation in order to clarify the relationship between the CDW and superconductivity. The results show a CDW incommensuration arising at an intercalation value coincident with the onset of superconductivity at around x=0.055(5). Additionally, it was found that the charge density wave persists to higher intercalant concentrations than previously assumed, demonstrating that the CDW does not terminate inside the superconducting dome. A charge density wave peak was observed in samples up to x=0.091(6), the highest copper concentration examined in this study. The phase diagram established in this work suggests that charge density wave incommensuration may play a role in the formation of the superconducting state.

Combined small and wide angle synchrotron x-ray scattering (SAXS and WAXS) techniques have been developed for in situ high pressure samples, enabling exploration of the atomic structure and nanoscale superstructure phase relations. These studies can then be used to find connections between nanoparticle surfaces and internal atomic arrangements. We developed a four-axis control system for the detector, which we then employed for the study of two supercrystals assembled from 5 nm Fe(3)O(4) and 10 nm Au nanoparticles. We optimized the x-ray energy and the sample-to-detector distance to facilitate simultaneous collection of both SAXS and WAXS. We further performed in situ high pressure SAXS and WAXS on a cubic supercrystal assembled from 4 nm wurtzite-structure CdSe nanoparticles. While wurtzite-structure CdSe nanoparticles transform into a rocksalt structure at 6.2 GPa, the cubic superstructure develops into a lamellarlike mesostructure at 9.6 GPa. Nanoparticle coupling and interaction could be enhanced, thus reducing the compressibility of the interparticle spacing above ∼3 GPa. At ∼6.2 GPa, the wurtzite-to-rocksalt phase transformation results in a noticeable drop of interparticle spacing. Above 6.2 GPa, a combined effect from denser CdSe nanoparticle causes the interparticle spacing to expand. These findings could be related to a series of changes including the surface structure, electronic and mechanical properties, and strain distribution of CdSe under pressure. This technique opens the way for exploring the new physics of nanoparticles and self-assembled superlattices.

Practically all synchrotron x-ray sources to data are based on the use of storage rings to produce the high current electron ͑or positron͒ beams needed for synchrotron radiation ͑SR͒. The ultimate limitations on the quality of the electron beam, which are directly reflected in many of the most important characteristics of the SR beams, arise from the physics of equilibrium processes fundamental to the operation of storage rings. It is possible to produce electron beams with superior characteristics for SR via photoinjected electron sources and high-energy linacs; however, the energy consumption of such machines is prohibitive. This limitation can be overcome by the use of an energy recovery linac ͑ERL͒, which involves configuring the electron-beam path to use the same superconducting linac as a decelerator of the electron beam after SR production, thereby recovering the beam energy for acceleration of new electrons. ERLs have the potential to produce SR beams with brilliance, coherence, time structure, and source size and shape which are superior to even the best third-generation storage ring sources, while maintaining flexible machine operation and competitive costs. Here, we describe a project to produce a hard x-ray ERL SR source at Cornell University, with emphasis on the characteristics, promise, and challenges of such an ERL machine.

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We establish that strong Fermi surface nesting drives the Néel transition in the RNi 2 Ge 2 compounds. Generalized susceptibility, x 0 ͑q͒, calculations found nesting to be responsible for both incommensurate wave vector, ͑0 0 0.793͒, in GdNi 2 Ge 2 , and the commensurate structure, ͑0 0 1͒, in EuNi 2 Ge 2 , as revealed by x-ray resonant exchange scattering. A continuous transition from incommensurate to commensurate magnetic structures via band filling is predicted. The surprisingly higher T N in EuNi 2 Ge 2 than that in GdNi 2 Ge 2 is also explained. PACS numbers: 75.10. -b, 75.25. + z, 75.30. -m, 78.70.CkRare-earth intermetallics with the tetragonal ThCr 2 Si 2 structure have been the subject of intensive study for several decades because of their intricate magnetic structures and various correlated electron phenomena [1]. The complex crystal structure and multiatom composition of these materials, relative to the elemental rare-earth metals, result in more involved band structures and magnetic interactions. While experimental studies of their magnetism have focused on the determination of ordered states, a quantitative theoretical understanding of their magnetic phase transition is lacking. In earlier work [2] on RNi 2 Ge 2 compounds it was noted that, at the onset of antiferromagnetic (AF) ordering in the Pr, Nd, Sm, and Tb through Tm members of the series, there is an incommensurate magnetic modulation of the form q ͑0 0 q z ͒ with q z in the range of ͑0.75 0.81͒ ͑ 2p c ͒, where c is the axial lattice parameter. The persistence of a single incommensurate q along the high-symmetry L line across the series is similar to the case of the elemental rare earths (Tb-Tm) [3]. In these intermetallic systems with low ordering temperatures ͑&30 K͒, the R atoms are well separated ͑*4 Å͒ from each other. Thus, the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction, via the conduction electron polarization, is believed to be responsible for their magnetic ordering.